Z DEPENDENCE OF L-SUBSHELL CROSS SECTIONS AROUND Z=50

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Z DEPENDENCE OF L-SUBSHELL CROSS SECTIONS AROUND Z=50

G. Spadaccini, P. Cuzzocrea, N. de Cesare, E. Perillo, M. Vigilante

To cite this version:

G. Spadaccini, P. Cuzzocrea, N. de Cesare, E. Perillo, M. Vigilante. Z DEPENDENCE OF L-

SUBSHELL CROSS SECTIONS AROUND Z=50. Journal de Physique Colloques, 1987, 48 (C9),

pp.C9-535-C9-538. �10.1051/jphyscol:1987988�. �jpa-00227409�

Z DEPENDENCE OF L-SUBSHELL CROSS SECTIONS AROUND Z=50

G. SPADACCINI, P. CUZZOCREA, N. DE CESARE, E. PERILLO and M. VIGILANTE

Dipartimento di Fisica Nucleare, Struttura della Materia e Fisica Applicata. Universita' di Napoli and Istituto Nazionale di Fisica Nucleate, Sezione di Napoli, Mostra dfOltremare, Pad 20, I-80125 Napoli, Italy

Abstract

Experimental results on L X-ray cross sections, obtained by 4He ion bombardment of thin targets for elements ranging from Z=46 to Z=55 and in the energy range EdHe= 0.6-5.0 MeV, have been compared with theoretical predictions.

The linear dependence of the logarithm of the production cross sections with respect to Z of the target element has been confirmed.

A comparison between the L and L , , 3 cross sections shows smooth dependence of the L1 fluorescence yield in relation to Z.

The analysis also shows a generally good agreement of the ionisation cross sections with the ECPSSR calculations.

Besides, no second order effects can be inferred from our data.

Introduction

Experimental information about the behaviour of the L-subshell vacancy decay parameters can be obtained by means of systematic measurements of L X-ray line production cross sections and from the L-subshell ionisation cross sections that can be deduced from the formers. This is specially interesting in those regions of the atomic number Z where discontinuities in some of these parameters are predicted /1-2/.

Furthermore, theoretical prediction of linear dependence on Z of the logarithm of the production cross sections / 3 - 4 / has been sometimes used to obtain interpolated values for those elements for which no direct measurements exist 1 5 1 .

Unfortunately, the extension of such measurements to elements with Z < < S O , is a difficult task when using Si(Li) detectors because the detector efficiency drops quite rapidly at the energies of the involved L X-ray lines and its finite energy resolution does not

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987988

JOURNAL DE PHYSIQUE

allow satisfactory line separation in order to extract the L- subshell ionisation cross sections.

Experiments

Our measurements were carried out at the Laboratori Nazionali di Legnaro (Padua) by using the aHe-+ ion beams of AN-2000 and CN Van de Graaf accelerators. Thin targets (-20 ug/cm2) were prepared by vacuum evaporation onto carbon backings (-10 ug/cm2). Further, the targets were covered with a thin copper layer (-5 ug/cm2) in order to get absolute X-ray production cross sections by normalizing to the K-shell cross sections of copper.

A detailed discussion about the experimental setup, including geometry, target preparation, detector calibration and efficiency measurements can be found in previous papers /6-7/. Similarly, in /6- 7/ and in references therein a discussion can be found about the analysis of the experimental spectra and the methods used in the reduction of the data.

Results discussion

Five X-ray groups (Ld, Lpl, 3.4.6

,

^{Lp2. 15, Lgl} ,

,

^{L ~ z .} 3) were identified in the experimental spectra and the related absolute production cross sections for elements ranging from Z=46 to Z=55 were reported in tabular form in /6-7/ at different incident beam energy

(EaHC ~0.6-5.0 MeV).

Figure 1 shows some of these data (particularly those concerning the L* and the Ld2.3 lines) plotted against Z and against the incident energy. The linear dependence on Z of the logarithm of the cross section is clearly confirmed experimentally for the L cross sections in this range of Z. The same result is obtained in the case of the Lp's and _{L y l ,} lines.

Concerning the Lx2.3 cross sections, one should note that when they are analysed in relation to the incident energy, there is a

"kink" (whose position depends on the Z value of the target element) due to the node in the 2 ~ wave function. Therefore, only for the ~ 1 ~ highest energies used in our measurements, one can observe the slope of the logarithm of the Ly2.3 cross sections in an unbiased way. In these cases there are no significant differences between the slopes vs. Z of In% and lnsLtz, 3. Since G x n , depends strongly on the variation of the parameter&= (fluorescence yield for LI), while GLa only slightly, this confirms that the parameterw~ has a smooth dependence upon Z /8-9/.

From the production cross sections we derived ionisation cross sections by the method of Datz et a1./10/. In a previous work /11/We discussed why this method was preferred over others. Fluorescence yields W L and Coster-Kronig rates f13 were taken from the compilation of Krause /12/ while radiative widths were taken from the relativistic calculations of Scofield 1 3 These parameters are calculated assuming single vacancy production, but we have already shown /9/ that they can still be used when significant NL, but moderate ML, multi- ionisation takes place.

The results were compared with the ECPSSR model / 4 / , the per- turbed stationary state theory with corrections for energy loss, Coulomb retardation and relativistic motion of the orbital electrons.

The values were obtained by using the tables of Choi et a1./14/.

theoretical values are plotted against the reduced velocity parameter

y,

^/3/.

With regard to the L1 and L3 subshells, for all the elements investigated in the specified range of Z , the theory gives an overall good agreement, better for E4,,>2 MeV

.

As far as it concerns the L1 subshell, in the same energy range the data are reasonably reproduced by the ECPSSR calculations only for those elements which are well out of the region of discontinuity of the atomic parameters around Z=50. In particular, for Ag, Cd and In the comparison shows large discrepancies, like the ones observed for protons as projectiles and reported elsewhere /15-16/. Such discrepan- cies can be attributed to an inadequacy of the atomic parameters in the region of the discontinuities associated with the cut-off of the Ll-LsMs,s Coster-Kronig transitions /1-2/.

The discrepancies are noticeable for all the investigated ele- ments at lower energies, close to the region of the "kink". They cannot be attributed /7,11/ either to trajectory effects /17/, or to vacancy rearrangement effects /18/. As matter of fact they should be attributed to the used way of inserting the corrections into the theories.

Clearly a more accurate way of calculating the various correc- tions would improve the agreement.

References

/1/ M.H. Chen, B. Crasemann, K.N. Huang, M. doyagi and R. Mark, At. Data Nucl. Data Tables 19 97 (1977)

/2/ M.H. Chen, B. Crasemann and R. Mark, Phys. Rev.

A24

177 (1981) /3/ W. Brandt and G. Lapicki, Phys. Rev.

A20

465 (1979)

/4/ W. Brandt and G. Lapicki, Phys. Rev. A 2 1717 (1981)

/5/ A. Pedro de Jesus, T.M. Pinheiro, I.A. Niza, J.P. Ribeiro and J.S. Lopes, Nucl. Instrum. Methods

B15

595 (1986)

/6/ E. Perillo, G. Spadaccini, M. Vigilante, P. Cuzzocrea and N. De Cesare, Nuovo Cim. 71 (1987)

/7/ E. Perillo, P. Cuzzocrea, N. De Cesare, G. Spadaccini and M. Vigilante, J. Phys. B:At. Mol. Phys. to be published /8/ B.L. Doyle and S.M. Shafroth, Phys. Rev.

A19

1433 (1979) /9/ E. Perillo, G. Spadaccini, M. Vigilante, P. Cuzzocrea and

N. De Cesare, J. Phys. B:At. Mol. Phys.

2

1275 (1987) /lo/ S. Datz, J.L. Duggan, L.C. Feldman, E. Laegsgaard and

J.U. Andersen, Phys. Rev. 192 (1974)

/11/ E. Perillo, G. Spadaccini, M. Vigilante and P. Cuzzocrea, J. Phys, B:At. Mol. Phys. 19 4161 (1986)

/12/ M.O. Krause, J. Phys. c h e m 7 ~ e f . Data

8

307 (1979) /13/ J.H. Scofield, At. Data Nucl. Data Tables

14

121 (1974) /14/ B.H. Choi, E. Merzbacher and G.S. Khandelwal,

At. Data

3

291 (1973)

/15/ E. Rosato, Phys. Rev.

A28

2759 (1983)

/16/ P. Cuzzocrea, E. Perillo, E. Rosato, G. Spadaccini and M. Vigilante, J. Phys. B:At. Mol. Phys

3

711 (1985) /17/ W. Jitschin, R. Hippler, K. Finck, R. Scach and L.O. Lutz,

J. Phys. B:At. Mol. Phys.

16

4405 (1983)

/18/ L. Sarkadi and T. Mukoyama, Nucl. Instrum. Methods 296 (1984) and references therein

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Figure 1

- eL4

(lower) and

CL.d,.,

(upper) versus Z

and the incident 4He ion energy. Data from /6-7/.

2

-

^{L 1}

-

i

^r

. . .

^{I - a .}

i . . -

0 " " I " ' I I I f a '

0.0 0.5 1.0 1.5

:,,

Figure 2

-

Comparison of the experimental L,-subshell ionisation cross sections with the ECPSSR predictions. A Pd; Cs.

Typical error bars are shown.